2 Implications of Miocene–Pliocene cooling in the India-Asia ......1 Low-temperature...
Transcript of 2 Implications of Miocene–Pliocene cooling in the India-Asia ......1 Low-temperature...
Low-temperature thermochronology of the Indus Basin in central Ladakh, northwest India: 1
Implications of Miocene–Pliocene cooling in the India-Asia collision zone 2
Gourab Bhattacharya1,2*, Delores M. Robinson1,2, Devon A. Orme3, Yani Najman4, Andrew 3
Carter5 4
1Department of Geological Sciences, The University of Alabama, AL-35487, USA 5
2Center for Sedimentary Basin Studies, The University of Alabama, AL-35487, USA 6
3Department of Earth Sciences, Montana State University, MT-59717, USA 7
4Lancaster Environment Centre, Lancaster University, LA-14YQ, UK 8
5Department of Earth & Planetary Sciences, Birkbeck, University of London, WC1E 7HX, UK 9
*Corresponding author: [email protected] 10
11
Abstract: The India-Asia collision zone in Ladakh, northwest India, records a sequence of 12
tectono-thermal events in the interior of the Himalayan orogen following the intercontinental 13
collision between India and Asia in early Cenozoic time. We present zircon fission-track, and 14
zircon and apatite (U-Th)/He thermochronometric data from the Indus Basin sedimentary rocks 15
that are exposed along the strike of the collision zone in central Ladakh. These data reveal a post-16
depositional Miocene–Pliocene (~22–4 Ma) cooling signal along the India-Asia collision zone in 17
northwest India. Our ZFT cooling ages indicate that maximum basin temperatures exceeded 200 18
°C but stayed below 280–300 °C in the stratigraphically deeper marine and continental strata. 19
Thermal modeling of zircon and apatite (U-Th)/He cooling ages suggests post-depositional basin 20
cooling initiated in Early Miocene time by ~22–20 Ma, occurred throughout the basin across zircon 21
(U-Th)/He partial retention temperatures from ~20–10 Ma, and continued in the Pliocene time 22
until at least ~4 Ma. We attribute the burial of the Indus Basin to sedimentation and movement 23
along the regional Great Counter thrust. The ensuing Miocene–Pliocene cooling resulted from 24
erosion by the Indus River that transects the basin. An approximately coeval cooling signal is well 25
documented east of the study area, along the collision zone in south Tibet. Our new data provide 26
a regional framework upon which future studies can explore the possible interrelationships 27
between tectonic, geodynamic and geomorphologic factors contributing to Miocene–Pliocene 28
cooling along the India-Asia collision zone from NW India to south Tibet. 29
30
Keywords: Indus Basin, exhumation, cooling, thermochronology, India-Asia collision zone, 31
Ladakh 32
1. Introduction 33
The India-Asia collision zone developed when the Neo-Tethyan ocean closed following 34
the continent-continent collision between India and Asia in early Cenozoic time (e.g., Searle, 2019; 35
Kapp and DeCelles, 2019). The sedimentary basins along the collision zone present a natural 36
laboratory to test models of deposition and exhumation in the interior of the Himalayan orogenic 37
system. The collision zone in Ladakh, northwest (NW) India exposes the Indus Molasse or the 38
Indus Basin sedimentary rocks (IBSR), which are a linear suite of deformed marine and continental 39
strata that were discontinuously deposited from Late Cretaceous to Pliocene time (Figures 1A–B; 40
Garzanti and Van Haver, 1988; Searle et al., 1990; Clift et al., 2002; Henderson et al., 2010; 2011). 41
Thus, the IBSR present an opportunity to study the pre- and syn-collisional tectono-thermal events 42
associated with the evolution of the intercontinental suture zone between India and Asia. Knowing 43
the timing and extent of suture zone basin exhumation is critical to understanding the surficial-to-44
lithospheric scale processes triggering it, which are intrinsically linked to the geological evolution 45
of the orogenic hinterland. 46
Previous studies on the thermal history of the IBSR along the collision zone in NW India 47
and on coeval rocks along the collision zone in south Tibet yield different exhumation histories. 48
Carrapa et al. (2014) present (U-Th)/He detrital zircon (ZHe) and apatite fission-track (AFT) 49
cooling ages from the Late Oligocene–Early Miocene Kailas Formation of the Yarlung suture in 50
south Tibet, which record basin exhumation from ~21–15 Ma. The authors interpret that these 51
cooling ages reflect incision by the paleo-Yarlung River as the Indian plate underthrusted beneath 52
Asia. In addition, Tremblay et al. (2015) and Orme (2019) document Early–Middle Miocene (~21–53
11 Ma) cooling in the Gangdese batholith and the Xigaze forearc basin in south Tibet, thereby 54
emphasizing that Miocene cooling along the India-Asia collision zone was a regional thermal 55
event. By contrast, in NW India, Tripathy-Lang et al. (2013) report ~52–28 Ma ZHe cooling ages 56
from the Kailas-contemporaneous Late Oligocene Basgo Formation. Unlike post-depositional 57
Miocene cooling as recorded in south Tibet, the ZHe cooling ages of the Basgo Formation in NW 58
India are interpreted to be unreset after deposition and are attributed to exhumation of the source 59
– the rapidly-eroding Indian margin (Tripathy-Lang et al., 2013). The only previously reported 60
evidence of post-depositional Miocene cooling in the IBSR is limited to two AFT ages of ~14–12 61
Ma (Clift et al., 2002) and a single AFT age of ~7 Ma (Schlup et al., 2003). However, ZFT, ZHe 62
and AFT ages from the Ladakh batholith to the north of the IBSR indicate rapid cooling along the 63
collision zone in NW India at ~26–18 Ma (Kirstein et al., 2006). 64
To determine if a Miocene cooling signal is present across different formations in the Indus 65
Basin in NW India, we sampled the IBSR across 4 traverses in central Ladakh: Temesgam and 66
Basgo sections in the west, Zanskar Gorge in the center, and Upshi-Lato section in the east (Figure 67
1A). We present ZFT, ZHe and (U-Th)/He detrital apatite (AHe) data to resolve the thermal history 68
of the IBSR and investigate the underlying causes that contributed to the heating and cooling along 69
the India-Asia collision zone in NW India. 70
2. Geologic Background 71
2.1 Tectonic Setting 72
From south to north, the India-Asia collision zone in NW India (Figures 1B–C) is 73
composed of: a) the Precambrian–Paleocene Greater Indian passive margin metasedimentary and 74
sedimentary rocks of the Tethyan Himalaya with an isolated klippe of the Cretaceous Spongtang 75
oceanic arc (Garzanti et al., 1987; Buckman et al., 2018), b) the Indus Suture Zone containing the 76
Lamayuru Complex – the Mesozoic deep-water slope facies of the Indian margin (Robertson and 77
Sharp, 1998), and the Dras-Nidar Complexes – an assemblage of Cretaceous ophiolitic mélange, 78
volcanic and volcano-sedimentary units (Ahmad et al., 2008; Walsh et al., 2019; Das et al., 2020), 79
c) the Late Cretaceous-Pliocene IBSR (Garzanti and Van Haver, 1988; Searle et al., 1990), and d) 80
the southern edge of the Early Cretaceous–Early Eocene Ladakh batholith (Weinberg and Dunlap, 81
2000). The IBSR unconformably overlies the Ladakh batholith to the north and are in fault contact 82
with the Dras-Nidar Complexes to the south (Figures 1A, C; Searle et al., 1990; St-Onge et al., 83
2010). Pre-collisional deposition of the IBSR initiated in an arc-bounded or forearc marine basin 84
in Late Cretaceous time, and the depocenter evolved into a continental intermontane basin with 85
the onset of India-Asia collision in Early Eocene time (Garzanti and van Haver, 1988; Henderson 86
et al., 2010). Regional IBSR deposition largely ended by Late Oligocene–Early Miocene time 87
(~26–23 Ma) when basin inversion began, although local-scale deposition continued in Pliocene 88
time in patches of western and central Ladakh (Mathur, 1983; Clift et al., 2002, Henderson et al., 89
2010, 2011; Zhou et al., 2020; Bhattacharya et al., 2020). 90
Structurally, the IBSR constitutes the footwall of the regional north-vergent Main Zanskar 91
backthrust (Searle et al., 1997), also known as the Great Counter thrust (GCT, Figure 1A). Multiple 92
strike-parallel, north-vergent thrusts belonging to the GCT system deform the IBSR (Steck, 2003). 93
The timing of movement along the GCT in NW India is indirectly constrained to 23–20 Ma on the 94
basis of the age of tectonic and metamorphic processes in the Himalayan orogen. Using 40Ar/39Ar 95
hornblende ages, Searle et al. (1992) determine that peak metamorphism (700–750 °C, 8 kbar) and 96
maximum crustal thickening in the Zanskar Himalaya (Figure 1A) south of the GCT occurred at 97
~28–23 Ma. Sinclair and Jaffey (2001) and Clift et al. (2002) suggest that this episode of crustal 98
thickening and uplift in the Himalayan wedge at ~28–23 Ma provided the mechanical force to 99
initiate movement along the GCT at ~23–20 Ma, thereby inverting the IBSR. Recent studies from 100
south Tibet, based on geochronology-thermochronology datasets and cross-cutting relationships 101
among Neogene intrusive rocks, also indicate that motion along the GCT initiated at ~23 Ma and 102
largely ended by ~15 Ma (Zhang et al., 2011; Carrapa et al., 2014; Laskowski et al., 2018; Orme, 103
2019). 104
2.2 Stratigraphy 105
The IBSR stratigraphy comprises two major rock groups (Table 1, Figures 1C, 2A-C): (a) 106
the southern Late Cretaceous–Early Eocene marine Tar Group (Figures 2B-C), and (b) the northern 107
Early Eocene to Pliocene continental Indus Group (Figure 2A-C). The Tar Group consists of 108
carbonate and siliciclastic rocks that are tectonically bounded to the south by the pre-collisional 109
Dras-Lamayuru-Nidar Complexes and are juxtaposed in the north against the Indus Group. The 110
Indus Group exhibits extreme along-strike variations in siliciclastic fluvial facies that 111
unconformably overlie the Ladakh batholith (Brookfield and Andrews-Speed, 1984; Garzanti and 112
Van Haver, 1988; Searle et al., 1990; Sinclair and Jaffrey, 2001; Clift et al., 2002; Steck, 2003; 113
Wu et al., 2007; St-Onge et al., 2010; Henderson et al., 2010, 2011; Tripathy-Lang et al., 2013; 114
Singh et al., 2015; Zhou et al., 2020; Bhattacharya et al., 2020). The Indus Group is categorized 115
into two sub-groups: (i) the Early Eocene–Early Miocene Lower Indus Group and (ii) the Pliocene 116
Upper Indus Group. The former is regionally present along the India-Asia collision zone, while 117
the latter is localized to central and far-western Ladakh (Mathur, 1983; Henderson, et al., 2010). 118
2.3 Previous Low-temperature Thermochronometric Studies 119
Low-temperature thermochronologic data and other thermal proxies from the IBSR are 120
limited to a few local studies with conflicting interpretations, leaving the regional thermal history 121
undetermined. K/Ar mica ages from phyllites of the Indus Basin indicate a low-grade anchizonal 122
metamorphic event along its southwestern margin in Middle–Late Eocene time, when fold-thrust 123
deformation occurred in the Tethyan Himalaya (Van Haver et al., 1986; Steck, 2003). Using illite 124
crystallinity and vitrinite reflectance, Van Haver (1984) determined peak basin temperatures of 125
~280 °C in the uppermost Tar Group (Nummulitic Limestone Formation, Figure 2B) and ~155 °C 126
in the Lower Indus Group. Clift et al. (2002) report 14–12 Ma AFT ages from two Lower Indus 127
Group samples and interpret that these ages reflect cooling following basin inversion associated 128
with regional counterthrusting along the GCT at ~23–20 Ma. Illite crystallinity estimates by Clift 129
et al. (2002) in central Ladakh suggests temperatures did not exceed 200 °C in the Indus Group. 130
Another paleo-geotemperature study from the Indus Group in eastern Ladakh by Schlup et al. 131
(2003), which is also discussed in Clift et al. (2004), reveals an illite crystallinity index of 0.36 132
(ºΔ2θ) from a single Lower Indus Group sandstone sample. This illite crystallinity value translates 133
to a lower anchizone grade burial temperature of ~239°C using the index-temperature equation of 134
Zhu et al. (2016). Schlup et al. (2003) also report a ZFT central age of 23 ± 2 Ma and an AFT age 135
of 7.4 ± 0.7 Ma from the same sandstone sample. The 23 ± 2 Ma ZFT age is interpreted to reflect 136
source cooling and is attributed to the exhumation of the Ladakh batholith, while the 7.4 ± 0.7 Ma 137
AFT age suggests post-depositional cooling in the basin. Tripathy-Lang et al. (2013) report unreset 138
ZHe ages of ~52–28 Ma in the Lower Indus Group Late Oligocene Basgo Formation and attribute 139
them to the exhumation of source regions on the Indian plate. 140
3. Sampling and Analytical Methods 141
3.1 Sampling 142
We sampled medium-grained sandstones from four N-S to NNE-SSW trending sections 143
across the IBSR in central Ladakh (Figures 1A, 2A-C). These sections are: a) Temesgam (Figure 144
2A), b) Basgo, (Figure 2A), c) Zanskar Gorge (Figure 2B) and d) Upshi-Lato (Figure 2C). We 145
collected eight samples from the Zanskar Gorge for ZFT analyses (Figure 2B). Low yield of good 146
quality dateable apatite in most samples and zircon in several samples limited our AHe (two 147
samples; Figures 2A, 2C) and ZHe datasets (six samples; Figures 2A-C). 148
Zircon and apatite concentrates were separated from each 8-10 kg sample using 149
conventional mineral separation techniques involving a rock crusher, water table, Frantz magnetic 150
separator and heavy liquids. Only samples DZA23TM from the Temesgam Formation (Figure 2A) 151
and DZA08UL (Figure 2C) from the Lower Upshi Formation produced apatites suitable for AHe 152
dating. Zircon yield in sample DZA08UL was low. 153
3.2. Zircon fission-track thermochronology 154
The zircons were mounted, polished and etched with KOH–NaOH at 220 °C for 12–36 155
hours following standard procedures of the London Fission Track Research Group. Mounts were 156
then irradiated with muscovite external detectors and dosimeter glass CN-5 and CN-2 at the 157
thermal neutron facility of the Risø reactor, Denmark. Fission-track densities were measured using 158
an optical microscope at 1250x magnification with an oil objective. Ages (±1σ) were calibrated by 159
the zeta method (Hurford and Green, 1983), using a zeta factor of 127 ± 5 that was determined 160
from multiple analyses of zircon standards following the recommendations of Hurford (1990). 161
ZFT ages indicate cooling through the 240 ± 40 °C temperature window depending on their 162
U-concentrations (Hurford, 1986). Ideally, if all the ZFT ages are younger or older than 163
depositional age of the basin, they indicate cooling in the basin or source, respectively. A mixture 164
of older and younger ages, spanning pre- and post-deposition ages, likely suggests a case of partial 165
fission-track annealing (or partial resetting) in zircon that indicates basin temperatures were within 166
240±40 °C. Partial annealing of zircon fission tracks begin at ~185–200 °C and the annealing is 167
complete above ~280–300 °C (Bernet and Garver, 2005). 168
3.3 (U-Th)/He Zircon and Apatite Thermochronology 169
At the Arizona Radiogenic Helium Dating Laboratory, 3–5 mostly inclusion-free zircon 170
and apatite grains with angular crystal faces were hand-picked from each sample (if available) and 171
packed into Nb tubes. Applying the standard procedures of He extraction using coupled laser 172
heating, the He content was measured on a quadrupole mass spectrometer, and subsequently Th 173
and U contents were measured using ICP-MS following the methods of Reiners (2005). Raw ages 174
were obtained by solving the combined radioactive decay-diffusion equation with known 175
analytical concentrations of U, Th and He. These raw ages were then corrected by applying the 176
alpha-ejection protocols of Farley et al. (1996). If the ZHe and AHe ages are younger than the 177
depositional age of the formation, this implies basin burial temperatures of >140–200 °C and >40–178
90 °C, respectively, and the ages are interpreted as thermally reset. ZHe or AHe ages that are older 179
than the depositional age of the sample are unreset and reflect cooling of the source before 180
deposition. 181
3.4 Thermal Modeling 182
The ZHe and AHe ages from each sample were inverse modelled in the thermal modeling 183
program HeFTy v.1.9.1 (Ketcham, 2005) to determine the time-Temperature (t-T) paths using the 184
diffusion model of Guenthner et al. (2013). The forward model in HeFTy predicts the expected 185
grain age data distribution for a given t-T path. The inverse algorithm solves for a family of t-T 186
paths that a sample could have experienced for a fixed input dataset that include cooling ages, U-187
Th-Sm concentrations, grain size and zonation parameters. For each resultant t-T path, HeFTy 188
calculates the statistical fit between measured and the predicted cooling ages. Acceptable-fit paths 189
have a Kolmogorov-Smirnov probability ≥ 0.05, while good-fit paths have a Kolmogorov-Smirnov 190
probability ≥ 0.5. A weighted mean path and a best-fit t-T path are also generated from the 191
inversion process. The weighted mean path is an overall summary of the inversion process with 192
weights based on goodness of fit statistics associated with the acceptable and good-fit paths; it may 193
or may not have an acceptable or a good fit to the data. The best-fit path has the highest goodness 194
of fit and represents the most reasonable thermal history of a sample under the assigned constraints. 195
4. Results 196
Our ZFT, ZHe, and AHe results are summarized in Table 2 and Figure 3 (data available in 197
the supporting information, Tables S1–S3). All ages are reported at 1𝜎 uncertainty level. For 198
previous studies that used the youngest single grain age (e.g., youngest single detrital zircon or 199
muscovite grain age) to constrain the maximum depositional ages (MDAs) of a unit, we 200
recalculated the MDA by estimating the weighted mean age of the youngest cluster with 201
overlapping uncertainties (i.e., YC1𝜎(2+) and YC2𝜎(3+) ages; Table 2, Dickinson and Gehrels, 202
2009). If true depositional age (e.g., biostratigraphic or geochronologic tuff ages) is not available 203
for a formation, the YC2𝜎(3+) age is adopted as a conservative estimate of its MDA (Coutts et al., 204
2019). Therefore, unless specified, a MDA reported in this study refers to the YC2𝜎(3+) age, 205
which is the weighted mean age of the youngest cluster of 3 or more grains with overlapping 2𝜎206
uncertainties. 207
The individual ZFT ages span from Cretaceous to Middle Miocene time, 182.15 ± 50.20 – 208
13.95 ± 2.98 Ma. Although the objective was to date 50–100 grains per sample for ZFT, low yield 209
of zircon resulted in 17–65 grains per sample. All eight samples from the Zanskar Gorge (Figures 210
2B, 3A) fail the χ2 test (P(χ2) < 5%) indicating the presence of some overdispersion amongst the 211
population of measured grain ages. However, levels of overdispersion expressed as % dispersion 212
of the central age are not always high, suggesting that in some cases, the overdispersion is not 213
significant or poorly developed in terms of defining discrete age components. The ZFT data were 214
decomposed into statistical grain-age components or modes using RadialPlotter (Vermeesch, 215
2009; Table 2); however, these do not necessarily capture the true age modes if represented by 216
only a few grains. In some cases, a few higher precision ages may be identified as an age mode 217
rather than a population of grains that capture the true Poisson age distribution. To help determine 218
the significance of the component ages, the data are also plotted as Abanico diagrams that combine 219
a radial plot and a probability density estimate (supporting information, Figure S1). These plots 220
help to visualize the distribution of ages in each sample in terms of age modes or groups, like the 221
youngest age mode, the secondary age mode and the oldest age mode. 222
The individual ZHe ages are all Miocene, 19.04 ± 0.54 – 8.57 ± 0.11 Ma (Table 2, Figure 223
3B). The AHe ages range from Late Miocene–Pliocene, 6.77 ± 0.40 – 3.94 ± 0.17 Ma (Table 2, 224
Figure 3B). 225
5. Interpretations 226
5.1 Tar Group and Lato Formation 227
The Tar Group, which has a biostratigraphically-determined depositional age limit of ~55–228
50 Ma (Green et al., 2008; Henderson et al., 2010), is partially reset with respect to the ZFT system, 229
with ages from Zanskar Gorge yielding 130.92 ± 31.87 to 21.53 ± 4.61 Ma for the lowermost 230
Jurutze Formation (sample ZG45), 76.79 ± 14.40 to 13.95 ± 2.98 Ma for the Chogdo Formation 231
(sample ZG55) in the middle, and 182.15 ± 50.20 to 22.60 ± 4.57 Ma for the topmost Nummulitic 232
Limestone Formation (sample ZG62). The zircon populations (or modes), which are younger than 233
the depositional ages in Chogdo (ZG55) and Nummulitic Limestone (ZG62) Formations confirm 234
partial resetting (Table 2, Figure 3A). Although sample ZG45 from the Jurutze Formation contains 235
a single mode of 65.2 ± 3.1 Ma, which is older than its 54.7 ± 0.3 Ma U-Pb detrital zircon MDA 236
(Table 2), this ZFT age likely reflects partial resetting within the PAZ whereby the older, inherited 237
zircons within the Jurutze Formation were not thermally reset to take them below the MDA. Our 238
data suggest that the burial temperatures in the Tar Group exceeded the ZFT lower partial 239
annealing temperatures of ~185-200 °C. However, basin temperatures did not exceed the higher 240
annealing temperatures of ~280-300 °C above which ZFT ages are completely reset. 241
The ZHe ages from the Tar Group Sumdo Formation (sample DZA20ZV; 15.42 ± 0.20 - 242
8.57 ± 0.11 Ma) in the Zanskar Gorge are all younger than its biostratigraphic age of ~55-51 Ma 243
(Henderson et al., 2010), indicating post-depositional temperatures exceeded 180-200 °C (Table 244
2, Figure 3B). The Lato Formation (possibly Cretaceous in age) within the Upshi-Lato transect is 245
older than the youngest units of the Tar Group and has ZHe ages (sample DZA12UL; 12.62 ± 0.26 246
- 10.05 ± 0.20 Ma) that are all considerably younger than its stratigraphic age (Table 2, Figure 3B). 247
Therefore, the Lato Formation is also reset. 248
5.2 Indus Group 249
In the Lower Indus Group at Zanskar Gorge, the 50.3 ± 3.3 Ma ZFT modal age of the Nurla 250
Formation (sample ZG42) is within error of its U-Pb detrital zircon MDA of ~51 Ma (Table 2, 251
Figure 3A; Bhattacharya et al., 2020). The Choksti Conglomerate (sample ZG38), which overlies 252
the Nurla Formation and is the basal member of the Choksti Formation, has two ZFT modes, 40.3 253
± 2.1 and 68.6 ± 4.1 Ma (Table 2, Figure 3A). These two age modes are approximately equal to or 254
older than the U-Pb detrital zircon MDA of the Choksti Formation, which is 41.5 ± 0.2 Ma (Wu 255
et al., 2007). The Upper Choksti member (sample ZG30), which is the topmost member of the 256
Choksti Formation, has four ZFT modes: 26.6 ± 2.2 (M1), 37.8 ± 3.8 (M2), 49.5 ± 4.4 (M3) and 257
83 ± 8.4 Ma (M4; Table 2, Figure 3A). The Upper Choksti Member is stratigraphically correlatable 258
to the Hemis and Lower Upshi Formations that have U-Pb detrital zircon MDAs of 37.8 ± 0.2 and 259
38.3 ± 0.2 Ma, respectively (Table 2; Sinclair and Jaffey, 2001; Henderson et al., 2011; 260
Bhattacharya et al., 2020). The Choksti Formation is also older than the Basgo Formation, which 261
has a Late Oligocene biostratigraphic age (Bajpai et al., 2004; Tripathy-Lang et al., 2013). The M1 262
mode of Upper Choksti is thus younger than its depositional age, reflecting partial resetting of 263
sample ZG30. Interestingly, partial resetting is not detected in samples ZG42 and ZG38 from the 264
underlying Nurla Formation and Choksti Conglomerate member. This is because these two 265
samples probably contained older zircon populations, which remained above their corresponding 266
MDAs, despite partial resetting. The youngest M1 mode from the Lower Nimu Formation (sample 267
ZG21) is 25.5 ± 3.1 Ma, which is younger than its 40Ar/39Ar detrital muscovite MDA of 32.3 ± 0.2 268
Ma. In terms of true depositional age, the Lower Nimu Formation is at least older than the 269
biostratigraphically-dated Late Oligocene Basgo Formation (Bajpai et al., 2004; Buckman et al., 270
2018). The M1 mode of the Lower Nimu Formation thus indicates partial resetting. The upper 271
Indus Group Upper Nimu Formation (ZG16) is unreset, with ZFT modes older than its 272
corresponding 40Ar/39Ar detrital muscovite MDA of 9.5 ± 0.5 Ma (Table 2, Henderson et al., 2010). 273
Overall, like the Tar Group, the Lower Indus Group is also partially reset with respect to the ZFT 274
system, whereas the Upper Indus Group is unreset. 275
Along the Upshi-Lato traverse, the Lower Indus Group Lower Upshi Formation (sample 276
DZA09UL) has ZHe ages from 17.79 ± 0.26 – 13.63 ± 0.21 Ma (Table 2, Figure 3B). The Lower 277
Upshi Formation and its stratigraphically correlatable Hemis Formation both have detrital zircon 278
and muscovite MDAs of ~38 Ma (Table 2; Henderson et al., 2011; Bhattacharya et al., 2020). The 279
ZHe ages in the Lower Upshi Formation are thus younger than its inferred MDA. Along the Basgo 280
traverse, the Lower Indus Group Basgo Formation (sample DZA07SA) ZHe ages are from 19.04 281
± 0.54 – 9.90 ± 0.27 Ma, which are younger than its Late Oligocene depositional age based on 282
ostracods (Bajpai et al., 2004). From the Temesgam traverse, the Lower Indus Group Temesgam 283
Formation (sample DZA23TM) exhibits ZHe ages from 18.91 ± 0.52 – 12.81 ± 0.18 Ma, which 284
are younger than its U-Pb detrital zircon MDA of 26.8 ± 0.1 Ma (Table 2; Bhattacharya et al., 285
2020). The AHe ages from the Lower Upshi Formation (sample DZA08UL; 6.56 ± 0.10 – 5.22 ± 286
0.30 Ma) and the Temesgam Formation (sample DZA23TM; 6.77 ± 0.40 Ma – 3.94 ± 0.17 Ma) 287
are younger than their corresponding ZHe ages (Table 2; Figure 3B). 288
All ZHe ages from the Lower Indus Group are <20 Ma. Deposition in the Lower Indus 289
Group of central Ladakh ended by ~26–23 Ma, after which basin inversion and regional 290
counterthrusting began at ~23–20 Ma (Clift et al., 2002; Zhou et al., 2020; Bhattacharya et al., 291
2020). Therefore, we interpret the ZHe and AHe ages from the Lower Indus Group formations as 292
thermally reset. This is consistent with our earlier interpretation that the Lower Indus Group is 293
partially reset with respect to the ZFT system, implying peak burial temperatures exceeded 185–294
200 °C but stayed below 280–300 °C. By contrast, the stratigraphically youngest Upper Indus 295
Group Upper Nimu Formation yields ZHe ages (sample DZA17ZV; 17.39 ± 0.35 – 13.70 ± 0.27 296
Ma) older than its corresponding 40Ar/39Ar detrital muscovite MDA of 9.5 ± 0.5 Ma (Henderson 297
et al., 2010). The Upper Indus Group is therefore unreset with respect to the ZHe system (Table 2, 298
Figure 3B). 299
No correlation exists between ZHe or AHe ages and grain size in individual samples. 300
However, compilation of all the ZHe ages reveals a moderate positive correlation between age and 301
grain size, which may contribute to the inter-sample ZHe age dispersion (supporting information, 302
Figure S2). No correlation exists between AHe ages and grain size. Overall, no correlation is 303
observed between effective uranium and ZHe or AHe ages within individual samples, or 304
collectively (supporting information, Figure S2). This suggests radiation damage is not the primary 305
influence of intra-sample ZHe and AHe age variability, and the distribution of ZHe ages are largely 306
geologically controlled. The only exception is sample DZA07SA from the Basgo Formation, 307
which shows strong negative correlation between ZHe age and effective uranium (R2 = ~0.7) 308
suggesting some control of radiation damage on the observed cooling ages (supporting 309
information, Figure S2). 310
6. Thermal modeling of (U-Th)/He cooling ages 311
6.1 Modeling Strategy 312
Using our ZHe and AHe data in the thermal modeling program HeFTy, we tested two t-T 313
modeling approaches to determine the cooling history of the Indus Basin rock samples. The first 314
approach involves considering post-depositional t-T constraints based on known regional geologic 315
information, while the second approach lacks any specific post-depositional t-T constraints. The 316
purpose of testing the second approach was to check if we can reproduce near-identical cooling 317
histories without imposing particular post-depositional t-T constraints in the models, thus reducing 318
bias. 319
Indus Basin sedimentation began in Late Cretaceous time with the deposition of the marine 320
Tar Group, which continued until ~50 Ma (Henderson et al., 2010). After ~50 Ma, the continental 321
facies of the Lower Indus Group were deposited until Late Oligocene–Early Miocene time 322
(Sinclair and Jaffey, 2001; Clift et al., 2002; Henderson et al., 2011; Zhou et al., 2020; 323
Bhattacharya et al., 2020). The Indus Basin was inverted at ~23–20 Ma (Clift et al., 2002) and 324
there is no prior evidence of post-depositional basin cooling. Burial temperatures largely remained 325
below 240 °C in the Indus Basin except the Tar Group, where maximum temperatures reached 280 326
°C (Van Haver, 1984; Clift et al., 2004). In our first approach, to fit the ZHe and AHe data in the 327
context of known regional geologic information, we allow individual models to explore the t-T 328
space younger than 23 Ma and colder than 240 or 280 °C (Figures 4A-E). We apply surface 329
depositional temperatures of 0–25 °C and let all the models solve for t-T paths from temperatures 330
greater than the closure temperature window of the warmest thermochronometric system 331
modelled. The ZFT, ZHe, and AHe partial annealing/retention temperatures considered are 240 ± 332
40 °C (Hurford, 1986), 140–200 °C (Reiners, 2005; Guenthner et al., 2013), and 40–90 °C (Ehlers 333
and Farley, 2003), respectively. Based on the knowledge of regional thermal history, a temperature 334
constraint of 0–280 °C was applied only to the Tar Group Sumdo Formation (Sections 2.3, 6.1.2), 335
while a 0–240 °C constraint was imposed on the t-T models of the Lato, Lower Upshi, Basgo and 336
Temesgam Formations (Sections 6.1.1, 6.1.3–6.1.5). The input t-T constraints are shown by hollow 337
rectangles in Figures 4A-E and are detailed for each formation in sections 6.1.1–6.1.5. For a given 338
sample, simultaneous modelling of individual ZHe ages, or a mix of individual ZHe and AHe ages 339
(2–3 grains or more), yielded no good or acceptable-fit paths with the known input data. This is a 340
common problem with HeFTy as noted in multiple previous studies (e.g., Carrapa et al., 2014); 341
the program could not satisfy all input parameters for a single sample simultaneously and produce 342
acceptable results. Therefore, mean ZHe and AHe ages were calculated and incorporated as input 343
data for t-T model extraction in HeFTy using the diffusion model of Guenthner et al. (2013). 344
Inverse modeling produced a set of possible t-T paths for a given sample based on the user assigned 345
t-T constraints. We ran the models until at least 100 good fit t-T paths were generated. The best-fit 346
t-T path of each model represents a statistically robust thermal history of the corresponding sample 347
(Figures 4A-E). 348
In the second approach of t-T modeling, we constrain only the depositional age of the 349
sample and its surface depositional temperatures (0–25 °C). This approach allows HeFTy to 350
explore maximum area in the post-depositional t-T space and generate a family of t-T paths that 351
do not depend on known geologic information from the region. Similar to first approach, at least 352
100 good fit t-T paths were produced (supporting information, Figure S3). Although the best-fit t-353
T paths from our second approach show cooling beginning approximately within the same age 354
range as in the first approach, not all the resultant t-T paths yield a geologically meaningful thermal 355
history. Several acceptable and good-fit paths demonstrate t-T histories that are unrealistic 356
considering the available data on the timing of basin sedimentation, burial, inversion and cooling. 357
Thus, not all statistically acceptable or good-fit t-T paths obtained in our second approach are 358
representative of the post-depositional cooling history of the basin. We examine the causes of 359
rejection for individual models in supporting information, Text S1. The second approach is not 360
discussed henceforth and the following sub-sections 6.1.1-6.1.5 focus on the t-T constraints 361
imposed by regional geologic data as per the first approach. 362
6.1.1 Lato Formation 363
The Indian margin unit Lato Formation was deposited on the surface at 0–25 °C in possibly 364
Cretaceous time (Figure 4A). The Lato Formation is speculated to be correlatable to the Mesozoic 365
Lamayuru Complex or the Mesozoic Chilling Formation in the Zanskar Gorge (Henderson et al., 366
2011), both of which are Indian margin units that are older than Early Eocene. Henderson et al. 367
(2011) obtained two ~51 and ~77 Ma U-Pb detrital zircon grain ages and a ~67 Ma 40Ar/39Ar 368
detrital muscovite grain age from the Lato Formation; all other detrital grains are >350 Ma. The 3 369
youngest grain ages do not overlap within 2𝜎; therefore, instead of taking a weighted average, we 370
consider the ~77 Ma grain age as a conservative estimate of MDA for the Lato Formation. The 371
Lato Formation is older than, or coeval with, the youngest Tar Group units that were deposited 372
between 55 and 50 Ma (Henderson et al., 2010, 2011). Therefore, in our HeFTy model, we 373
constrain the depositional age of the Lato Formation from ~77–50 Ma, which is consistent with 374
regional stratigraphic correlations. 375
Cooling is constrained through 0–240 °C after ~23 Ma. Despite being older than the Tar 376
Group, there is no evidence of burial temperatures exceeding 240 °C in the Lato Formation, and 377
the depositional setting of the Lato Formation relative to Tar Group is undetermined. The Tar 378
Group, which experienced temperatures >240 °C, has blue-grey phyllite (Van Haver, 1984; Clift 379
et al., 2002; Henderson et al., 2010) and was probably deposited just north of the Lato Formation 380
that contains relatively unaltered sandstone. 381
6.1.2 Sumdo Formation 382
The Tar Group Sumdo Formation was deposited at the surface (0–25 °C) at ~55–51 Ma 383
(Figure 4B; Henderson et al., 2010). ZFT ages from the overlying Chogdo Formation and the 384
underlying Jurutze Formation are partially reset, which suggest peak burial temperatures between 385
200–280 °C in the Sumdo Formation. Van Haver (1984) calculated a maximum burial temperature 386
of ~280 °C using illite crystallinity from the overlying Nummulitic Limestone Formation. 387
Therefore, we constrain cooling in the Sumdo Formation after 23 Ma through 0–280 °C. 388
6.1.3 Lower Upshi Formation 389
The Lower Indus Group Lower Upshi Formation (Figure 4C) is correlatable to the Hemis 390
Formation, and both have detrital zircon and muscovite MDAs of ~38 Ma (Henderon et al., 2011; 391
Singh et al., 2015; Bhattacharya et al., 2020). The 40Ar/39Ar detrital muscovite MDA of the Upper 392
Upshi Formation, which overlies the Lower Upshi Formation, is ~25 Ma (Table 2; Henderson et 393
al., 2011). Because true depositional ages can be younger than MDAs, we relax the depositional 394
age for the Lower Upshi Formation in our HeFTy model to be from ~38–23 Ma. The upper age of 395
~23 Ma is based on the ~26–23 Ma cessation of Lower Indus Group deposition in central Ladakh, 396
after which regional counterthrusting began at ~23–20 Ma (Clift et al., 2002; Zhou et al., 2020; 397
Bhattacharya et al., 2020). Our ZFT results indicate that the Lower Indus Group is partially reset 398
with respect to the ZFT system, indicating peak burial temperatures >185–200 °C. In addition, 399
paleo-geotemperature estimates from the Lower Indus Group based on illite crystallinity also 400
suggest maximum burial temperatures of ~239°C (Schlup et al., 2003; Clift et al., 2004). Hence, 401
we allow the model to cool through 0–240 °C after ~23 Ma. 402
6.1.4 Basgo Formation 403
The Lower Indus Group Basgo Formation is ~10–200 m thick (Garzanti and Van Haver, 404
1988) and is biostratigraphically dated as Late Oligocene in age (Bajpai et al., 2004). The 405
formation has a youngest single zircon MDA of ~27 Ma (Bhattacharya et al., 2020). The Basgo 406
Formation is conformably overlain by the Temesgam Formation, which was deposited from 26–407
23 Ma (Bhattacharya et al., 2020). In our t-T model, we constrain the depositional age of the Basgo 408
Formation at ~28–26 Ma (Figure 4D). Because Lower Indus Group temperatures did not exceed 409
240 °C, we constrain model cooling through 0–240 °C after ~23 Ma. 410
6.1.5 Temesgam Formation 411
The Lower Indus Group Temesgam Formation has a U-Pb detrital zircon MDA of ~27 Ma 412
and was deposited conformably on top of Basgo Formation from 26–23 Ma (Table 2, Bhattacharya 413
et al., 2020). Therefore, in our t-T model, we constrain the depositional age of the Temesgam 414
Formation from ~26–23 Ma (Figure 4E). An upper age limit of ~23 Ma is imposed from the 415
estimated age of inversion of the Indus Basin (Clift et al., 2002). Like other formations of the 416
Lower Indus Group, we allow model cooling through 0–240 °C after 23 Ma. 417
6.2 Model Results 418
All the t-T models demonstrate cooling from above or within the ZHe partial retention zone 419
temperatures of 140–200°C through at least 100 good and ≥188 acceptable-fit paths (Figures 4A–420
E). The best-fit t-T model paths show the onset of cooling by ~22–20 Ma in the Lower Indus Group 421
Lower Upshi, Basgo and Temesgam Formations (Figures 4C–E), and by ~15–13 Ma in the Lato 422
and Sumdo Formations (Figure 4A–B). It is possible that cooling may have started earlier than the 423
time indicated by the best-fit t-T paths in the Lato and Sumdo Formations as well; a number of 424
good-fit paths in each model suggest cooling began before ~15–13 Ma (Figures 4A–B). We 425
interpret the time of initiation of cooling along the best-fit t-T path as the minimum time by which 426
cooling was onset in the sample. The best-fit model paths for the Indian margin Lato Formation 427
and the Tar Group Sumdo Formation, demonstrate a peak burial temperatures (235–245 °C) well 428
exceeding the maximum ZHe partial retention zone temperature of ~200 °C, suggesting that the 429
Lato and Sumdo Formations are reset and the ZHe ages reflect post-depositional basin cooling 430
(Figures 4A–B). The Lower Upshi, Basgo and Temesgam Formations are likely reset as well; the 431
best-fit t-T model paths record cooling from above 170–190 °C, which indicate burial within the 432
higher side of the ZHe partial retention zone. Our t-T modeling is a consequence of using mean 433
ages in each model. If individual ZHe ages are modelled grain by grain, it does not significantly 434
change the results determined by using mean ages, and best-fit paths still indicate cooling 435
beginning between ~22 and 11 Ma. In summary, the t-T modeling results presented in this study 436
confirm the presence of a post-depositional cooling signal in the Indus Basin beginning at ~22–20 437
Ma, and show that burial temperatures in the Indian margin Lato Formation, Tar Group and the 438
Lower Indus Group exceeded 170–190 °C. 439
7. Discussion 440
7.1 Post-depositional Thermal Evolution of the IBSR 441
In general, the IBSR in central Ladakh, excluding the Upper Indus Group, experienced 442
post-depositional cooling from >170–200 °C in Miocene–Pliocene time. The ZFT results suggest 443
that post-depositional peak basin temperatures exceeded 185–200 °C in the Tar and Lower Indus 444
Groups but stayed below 280–300 °C (Table 2). This basin heating resulted in partial resetting of 445
the Tar and Lower Indus Group rocks with respect to the ZFT system. Our ZFT age interpretations 446
are consistent with the 280 °C and 240 °C maximum burial temperatures of the Tar and Lower 447
Indus Group rocks determined using illite crystallinity and/or vitrinite reflectance (Van Haver, 448
1986; Schlup et al., 2003, Clift et al., 2004). Although best-fit (U-Th)/He t-T model paths from the 449
Lower Indus Group suggest burial temperatures of ~170–190 °C, this might be a consequence of 450
relative extent of burial in the sampled sections. The Zanskar section, from where our ZFT samples 451
are collected, exposes more altered sandstones (Tripathy-Lang et al., 2013) compared to the Upshi-452
Lato, Basgo, and Temsgam sections, from where our Lower Indus Group ZHe and/or AHe samples 453
are collected. 454
Our ZHe ages range between ~19 and 8 Ma (Table 2, Figure 3B); however, these ages 455
alone cannot be used to estimate when basin cooling began. Thermal modeling results suggest that 456
cooling initiated by ~22–20 Ma in the Lower Indus Group of the Indus Basin (Figures 4C–E) and 457
was occurring throughout the basin by ~15–12 Ma (Figures 4A–B). The majority of the ZHe 458
cooling ages are between ~16 and 10 Ma, and all our thermal models demonstrate steady or rapid 459
cooling through 200–140 °C between ~20 and 10 Ma (Figure 4). Therefore, we suggest that 460
cooling largely occurred through ZHe temperatures in Early–Middle Miocene time. Cooling 461
continued into the Pliocene time until at least ~4 Ma, which is supported by our ~7–4 Ma AHe 462
cooling ages and model paths (Table 2, Figures 4C, E). Our interpretation expands the ~14–7 Ma 463
post-depositional cooling phase previously identified in the Lower Indus Group using three AFT 464
central ages (Clift et al., 2002; Schlup et al., 2003). It is also possible that the timing of initiation 465
of cooling decreases from north to south across the basin. For example, cooling may have begun 466
earlier in the northern Lower Indus Group Formations between ~22 and 20 Ma (Figures 4C–E), 467
and then progressed southwards in the Tar Group and Lato Formation between ~15-12 Ma (Figures 468
4A–B); however more low-temperature thermochronometric studies are required in the region to 469
check for such age trends across the Indus suture. Overall, this study in the Indus Basin of central 470
Ladakh reveals a post-depositional Miocene–Pliocene cooling phase (~22–4 Ma) that initiated at 471
~22–20 Ma. 472
Unreset ~17–14 Ma ZHe ages from the Pliocene Upper Nimu Formation (Table 2; Mathur, 473
1983; Henderson, 2010) of the stratigraphically youngest Upper Indus Group indicate post-474
depositional basin temperatures <140 °C. The Upper Indus Group is ~1 km thick (Henderson et 475
al., 2010). Therefore, Pliocene deposition of the Upper Indus Group did not influence the cooling 476
of either the Tar Group or the Lower Indus Group. 477
7.2 Cause of Basin Burial: Sedimentation or Overthrusting 478
In the Indus Basin, peak burial temperatures exceeded 170–190 °C just before cooling 479
began between ~22 and 20 Ma (Figure 4A–E). This requires the IBSR, excluding the Upper Indus 480
Group, to be progressively buried by sedimentation and/or regional overthrusting. Stratigraphic 481
studies indicate at least ~4.5 km of sediment was deposited in the Indus Basin by Early Miocene 482
time (Henderson et al., 2010; Bhattacharya et al., 2020), which suggests some of the basin heating 483
was the result of this stratigraphic overburden (assuming a geotherm of 20–30 °C/km). We suggest 484
that additional burial was caused by regional overthrusting associated with the GCT. Although the 485
age of the GCT is not well constrained by geochronological methods in NW India, it is thought to 486
have initiated in Early Miocene time at ~23–20 Ma (Sinclair and Jaffey, 2001; Clift et al., 2002; 487
discussed in Section 2.1). Kirstein et al. (2009) support a >20 Ma age for the GCT that led to the 488
burial of the southern edge of the Ladakh batholith. Recent studies from south Tibet also assert 489
that the slip on the GCT initiated at ~23 Ma (Laskowski et al., 2018), and ceased by ~15 Ma in 490
most locations (Zhang et al., 2011; Carrapa et al., 2014; Laskowski et al., 2018; Orme, 2019). 491
7.3 Implications and causes of cooling 492
Despite the relatively limited scope of our data, this is the first regionally extensive multi-493
thermchronometric study from the IBSR and reveals a post-depositional Miocene–Pliocene 494
cooling signal along the India-Asia collision zone in NW India. Deposition continued regionally 495
along the collision zone until Late Oligocene–Early Miocene time (~26–23 Ma; Sinclair and 496
Jaffey, 2001; Clift et al., 2002; Zhou et al., 2020), and there is no unequivocal evidence of cooling 497
beginning in the IBSR until ~22–20 Ma. Using the ZHe and AHe datasets, we calculate the amount 498
of material removed since the onset of cooling at ~22–20 Ma. This requires assuming a paleo-499
geothermal gradient, which is challenging considering the few studies along the collision zone in 500
NW India. Thermal modeling of ZFT and AFT ages in Kohistan, >350 km west of the study area, 501
reveal Miocene geothermal gradients of ~40 °C/km (Zeitler, 1985). Based on the geothermal 502
gradient calculated by Zeitler (1985), Sinclair and Jaffey (2001) bracket a 30–50 °C/km range for 503
Miocene geothermal gradients in the Indus Basin to estimate exhumation rates of 0.10–0.40 504
mm/yr. However, a 30–50 °C/km geothermal gradient range is incompatible with recent studies 505
from the region (e.g., Epard and Steck, 2008; Schlup et al., 2011; Langille et al., 2014; Kumar et 506
al., 2017). Using a bootstrapping algorithm, Kumar et al. (2017) modelled a range of geothermal 507
gradients from ~22–33 °C/km for the Early–Middle Eocene evolution of the Ladakh batholith 508
(Figure 1A) in NW India. In the Tso Morari Complex to the south (Figure 1A), Eocene–Oligocene 509
geothermal gradients were 18–22 °C/km, and the geothermal gradient has remained relatively 510
unperturbed since 30 Ma (Epard and Steck, 2008; Schlup et al., 2011). East of the Tso Morari 511
Complex, ~200 km south-east of the study area, Early Miocene geothermal gradients estimated 512
from the Leo Pargil shear zone by analyzing the Barrovian metamorphic pressure-temperature 513
paths vary from ~22–30 °C/km (Langille et al., 2014). Based on these neighboring geotherm 514
estimates, we assume a Miocene geothermal gradient of ~20–30 °C/km for the Indus Basin. It is 515
essential to note that recent works from sedimentary basins along the India-Asia collision zone in 516
south Tibet have all considered Miocene geothermal gradients within 20–30 °C/km (e.g., Carrapa 517
et al., 2014; Li et al., 2016; Orme, 2019; Ning et al., 2019). Assuming a geothermal gradient of 518
20–30 °C/km, our ZHe cooling ages indicate cooling from a mean temperature of 204 °C that 519
requires removal of at least ∼7–10 km of rock since ~22 Ma. 520
A potential driver of the Miocene–Pliocene cooling is erosion by the Indus River, which 521
has been draining the India-Asia collision zone in NW India since at least Late Oligocene–Early 522
Miocene time (Sinclair and Jaffey, 2001; Henderson et al., 2010, 2011; Bhattacharya et al., 2020). 523
Indus River erosion removed the GCT-overthrusted rocks that buried the Indus Basin, thereby 524
resulting in the observed Miocene–Pliocene cooling. Although Indus River erosion played an 525
important role in removing rocks from the India-Asia collision zone in Miocene–Pliocene time 526
(e.g., Sinclair and Jaffey, 2001; Henderson et al., 2010), we cannot be certain that the river erosion 527
was the primary factor triggering the onset of cooling between ~22 and 20 Ma. There is 528
considerable debate as to whether the Indus River’s flow along the suture zone began in NW India 529
in Early Eocene or Early Miocene time (Searle et al., 1996; Sinclair and Jaffey, 2001; Clift et al., 530
2002; Najman, 2006; Henderson et al., 2010; 2011; Zhuang et al., 2015). If the Indus River first 531
flowed along the suture zone in the Early Miocene, aggressive erosion resulting from its initiation 532
may explain the onset of regional cooling. If the Indus River existed at this location since Early 533
Eocene time, additional tectonic, geodynamic and geomorphological factors were also responsible 534
for the initiation of cooling. Interestingly, along the Yarlung suture of the India-Asia collision zone 535
in south Tibet, a regional Miocene cooling signal from ~21–7 Ma is well documented from low-536
temperature thermochronometric studies (e.g., Carrapa et al., 2014; Tremblay et al., 2015, Li et 537
al., 2015, 2016, 2017; Ge et al., 2017; Orme, 2019). These studies generally attribute the Miocene 538
cooling signal to GCT activity and/or Yarlung River erosion (Carrapa et al., 2014; Li et al., 2015, 539
2016, 2017; Ge et al., 2017; Orme, 2019), or intensification of Asian monsoon (Carrapa et al., 540
2014), while considering the regional uplift caused by the northward underthrusting of the Indian 541
plate following Greater Indian slab break-off in Early Miocene time (DeCelles et al., 2011; Webb 542
et al., 2017). Therefore, it is possible that a similar combination of tectonic, geodynamic, and 543
geomorphologic factors resulted in a tectonic setting that facilitated regional cooling along the 544
India-Asia collision zone in NW India. However, given the limited previously published and new 545
data in this region, it is difficult to test such scenarios. This study therefore provides the foundation 546
to investigate more complex tectono-thermal events in the India-Asia collision zone of NW India 547
and test models that correlate them with the results from south Tibet. 548
8. Conclusions 549
Low-temperature thermochronology of the Indus Basin in central Ladakh reveals a post-550
depositional Miocene–Pliocene (~22–4 Ma) cooling history. Our ZFT and ZHe results confirm 551
that the basin was buried to temperatures >170–200 °C and exceeded 240 °C in the deepest 552
formations. Basin burial is attributed to sedimentation and regional northward counterthrusting by 553
the GCT in Early Miocene time. Thermal modeling of ZHe and AHe ages indicate cooling onset 554
by ~22–20 Ma, occurred rapidly or steadily across the basin through ZHe partial retention zone 555
temperatures between ~20 and 10 Ma, and continued at least until ~4 Ma. This Miocene–Pliocene 556
cooling, which removed ~7–10 km of rock from the India-Asia collision zone, may be linked to 557
erosion by the Indus River that dissects the ISBR. However, more low-temperature 558
thermochronometric data from western and eastern Ladakh are required to confirm if this cooling 559
signal is present along the strike of the India-Asia collision zone in NW India, as documented in 560
south Tibet. If a regional Miocene–Pliocene cooling signal is indeed present both in NW India and 561
south Tibet, it might be indicative of a continental-scale thermal event operating along the India-562
Asia collision zone driven by a combination of tectonic, geodynamic, and geomorphologic factors 563
rather than Indus river incision alone. 564
Acknowledgements 565
We thank the Editor-in-Chief, Dr. Taylor Schildgen for editorial handling and valuable 566
feedback. Constructive reviews from Dr. Jean-Luc Epard and Dr. Ryan Leary greatly improved 567
the manuscript. All thermochronological data are available in the Supporting Information file and 568
can also be accessed at the 4TU.Centre for Research Data Repository via doi: 10.4121/12771332. 569
Student travel grants to GB by the Geological Society of America and The University of Alabama 570
funded the fieldwork for this research. Uttam Chowdhury, Erin Abel and Peter Reiners at the 571
Arizona Radiogenic Helium Dating Laboratory, The University of Arizona, assisted GB with the 572
He-analyses. Talat Ahmad, University of Kashmir helped with procuring permits for fieldwork. 573
Konchok Dorjay provided logistical support to GB in extreme weather conditions. 574
References 575
Ahmad, T., Tanaka, T., Sachan, H. K., Asahara, Y., Islam, R., & Khanna, P. P. (2008). 576 Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic 577 Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus 578 suture zone. Tectonophysics, 451(1-4), 206-224. 579 https://doi.org/10.1016/j.tecto.2007.11.049 580
Bajpai, S., Whatley, R. C., Prasad, G. V. R., & Whittaker, J. E. (2004). An Oligocene non-581 marine ostracod fauna from the Basgo Formation (Ladakh Molasse), NW Himalaya, 582 India. Journal of Micropalaeontology, 23(1), 3-9. https://doi.org/10.1144/jm.23.1.3 583
Bhattacharya, G., Robinson, D. M., Wielicki, M.M. (2020). Detrital zircon provenance of the 584 Indus Group, Ladakh, NW India: Implications for the timing of India-Asia collision and 585 other syn-orogenic processes. Geological Society of America Bulletin (Accepted, in copy-586 editing) 587
Bernet, M., & Garver, J. I. (2005). Fission-track analysis of detrital zircon. Reviews in 588 Mineralogy and Geochemistry, 58(1), 205-237. https://doi.org/10.2138/rmg.2005.58.8 589
Brookfield, M., & Andrews-Speed, C. (1984). Sedimentology, petrography and tectonic 590 significance of the shelf, flysch and molasse clastic deposits across the Indus suture zone, 591 Ladakh, NW India. Sedimentary Geology, 40(4), 249-286. https://doi.org/10.1016/0037-592 0738(84)90011-3 593
Buchs, N., & Epard, J.-L. (2018). Geology of the eastern part of the Tso Morari nappe, the Nidar 594 Ophiolite and the surrounding tectonic units (NW Himalaya, India). Journal of Maps, 595 15(2), 38-48. https://doi.org/10.1080/17445647.2018.1541196 596
Buckman, S., Aitchison, J. C., Nutman, A. P., Bennett, V. C., Saktura, W. M., Walsh, J. M. J., . . 597 . Hidaka, H. (2018). The Spongtang Massif in Ladakh, NW Himalaya: An Early 598 Cretaceous record of spontaneous, intra-oceanic subduction initiation in the Neotethys. 599 Gondwana Research, 63, 226-249. https://doi.org/10.1016/j.gr.2018.07.003 600
Carrapa, B., Orme, D., DeCelles, P. G., Kapp, P., Cosca, M. A., & Waldrip, R. (2014). Miocene 601 burial and exhumation of the India-Asia collision zone in southern Tibet: Response to 602 slab dynamics and erosion. Geology, 42(5), 443-446. https://doi.org/10.1130/G35350.1 603
Clift, P., Schlup, M., Carter, A., & Steck, A. (2004). Discussion of exhumation history of eastern 604 Ladakh revealed by 40Ar/39Ar and fission track ages: the Indus River–Tso Morari 605 transect, NW Himalaya. Journal of the Geological Society, 161(5), 893-894. 606 https://doi.org/10.1144/0016-764903-104 607
Clift, P. D., Carter, A., Krol, M., & Kirby, E. (2002). Constraints on India-Eurasia collision in 608 the Arabian Sea region taken from the Indus Group, Ladakh Himalaya, India. In P. D. 609 Clift, D. Kroon, C. Gaedicke, & J. Craig (Eds.), The Tectonic and Climatic Evolution of 610 the Arabian Sea Region: Geological Society, London, Special Publication 195 (pp. 97-611
116). https://doi.org/10.1144/gsl.Sp.2002.195.01.07 612
Coutts, D. S., Matthews, W. A., & Hubbard, S. M. (2019). Assessment of widely used methods 613 to derive depositional ages from detrital zircon populations. Geoscience Frontiers, 10(4), 614 1421-1435. https://doi.org/10.1016/j.gsf.2018.11.002 615
Das, S., Basu, A. R., Mukherjee, B. K., Marcantonio, F., Sen, K., Bhattacharya, S., & Gregory, 616 R. T. (2020). Origin of Indus ophiolite-hosted ophicarbonate veins: Isotopic evidence of 617 mixing between seawater and continental crust-derived fluid during Neo-Tethys closure. 618 Chemical Geology, 551, 119772. 619 https://doi.org/https://doi.org/10.1016/j.chemgeo.2020.119772 620
DeCelles, P. G., Kapp, P., Quade, J., & Gehrels, G. E. (2011). Oligocene-Miocene Kailas basin, 621 southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung 622 suture zone. Geological Society of America Bulletin, 123(7-8), 1337-1362. 623 https://doi.org/10.1130/b30258.1 624
Dickinson, W. R., & Gehrels, G. E. (2009). Use of U–Pb ages of detrital zircons to infer 625 maximum depositional ages of strata: A test against a Colorado Plateau Mesozoic 626 database. Earth and Planetary Science Letters, 288(1-2), 115-125. 627 https://doi.org/10.1016/j.epsl.2009.09.013 628
Dumitru, T. A. (1991). Effects of subduction parameters on geothermal gradients in forearcs, 629 with an application to Franciscan Subduction in California. Journal of Geophysical 630 Research: Solid Earth, 96(B1), 621-641. https://doi.org/10.1029/90jb01913 631
Ehlers, T. A., & Farley, K. A. (2003). Apatite (U–Th)/He thermochronometry: methods and 632 applications to problems in tectonic and surface processes. Earth and Planetary Science 633 Letters, 206(1-2), 1-14. https://doi.org/10.1016/S0012-821X(02)01069-5 634
Epard, J.-L., & Steck, A. (2008). Structural development of the Tso Morari ultra-high pressure 635 nappe of the Ladakh Himalaya. Tectonophysics, 451(1-4), 242-264. 636 https://doi.org/10.1016/j.tecto.2007.11.050 637
Farley, K., Wolf, R., & Silver, L. (1996). The effects of long alpha-stopping distances on (U� 638 Th)/He ages. Geochimica et cosmochimica acta, 60(21), 4223-4229. 639 https://doi.org/10.1016/S0016-7037(96)00193-7 640
Garzanti, E., Baud, A., & Mascle, G. (1987). Sedimentary record of the northward flight of India 641 and its collision with Eurasia (Ladakh Himalaya, India). Geodinamica Acta, 1(4/5), 297-642 312. https://doi.org/10.1080/09853111.1987.11105147 643
Garzanti, E., & Van Haver, T. (1988). The Indus clastics: forearc basin sedimentation in the 644 Ladakh Himalaya (India). Sedimentary Geology, 59(3-4), 237-249. 645 https://doi.org/10.1016/0037-0738(88)90078-4 646
Ge, Y.-K., Dai, J.-G., Wang, C.-S., Li, Y.-L., Xu, G.-Q., & Danisik, M. (2017). Cenozoic 647 thermo-tectonic evolution of the Gangdese batholith constrained by low-temperature 648
thermochronology. Gondwana Research, 41, 451-462. 649 https://doi.org/10.1016/j.gr.2016.05.006 650
Green, O. R., Searle, M. P., Corfield, R. I., & Corfield, R. M. (2008). Cretaceous-Tertiary 651 Carbonate Platform Evolution and the Age of the India-Asia Collision along the Ladakh 652 Himalaya (Northwest India). The Journal of Geology, 116(4), 331-353. 653 https://doi.org/10.1086/588831 654
Guenthner, W. R., Reiners, P. W., Ketcham, R. A., Nasdala, L., & Giester, G. (2013). Helium 655 diffusion in natural zircon: Radiation damage, anisotropy, and the interpretation of zircon 656 (U-Th)/He thermochronology. American Journal of Science, 313(3), 145-198. 657 https://doi.org/10.2475/03.2013.01 658
Henderson, A. L., Najman, Y., Parrish, R., BouDagher-Fadel, M., Barford, D., Garzanti, E., & 659 Andò, S. (2010). Geology of the Cenozoic Indus Basin sedimentary rocks: 660 Paleoenvironmental interpretation of sedimentation from the western Himalaya during 661 the early phases of India-Eurasia collision. Tectonics, 29(6), n/a-n/a. 662 https://doi.org/10.1029/2009tc002651 663
Henderson, A. L., Najman, Y., Parrish, R., Mark, D. F., & Foster, G. L. (2011). Constraints to 664 the timing of India–Eurasia collision; a re-evaluation of evidence from the Indus Basin 665 sedimentary rocks of the Indus–Tsangpo Suture Zone, Ladakh, India. Earth-Science 666 Reviews, 106(3-4), 265-292. https://doi.org/10.1016/j.earscirev.2011.02.006 667
Hurford, A. J. (1986). Cooling and uplift patterns in the Lepontine Alps South Central 668 Switzerland and an age of vertical movement on the Insubric fault line. Contributions to 669 Mineralogy and Petrology, 92(4), 413-427. https://doi.org/10.1007/BF00374424 670
Hurford, A. J. (1990). Standardization of fission track dating calibration: Recommendation by 671 the Fission Track Working Group of the I.U.G.S. Subcommission on Geochronology. 672 Chemical Geology: Isotope Geoscience section, 80(2), 171-178. 673 https://doi.org/10.1016/0168-9622(90)90025-8 674
Hurford, A. J., & Green, P. F. (1983). The zeta age calibration of fission-track dating. Chemical 675 Geology, 41, 285-317. https://doi.org/10.1016/S0009-2541(83)80026-6 676
Kapp, P., & DeCelles, P. G. (2019). Mesozoic–Cenozoic geological evolution of the Himalayan-677 Tibetan orogen and working tectonic hypotheses. American Journal of Science, 319(3), 678 159-254. https://doi.org/10.2475/03.2019.01 679
Ketcham, R. A. (2005). Forward and Inverse Modeling of Low-Temperature 680 Thermochronometry Data. Reviews in Mineralogy and Geochemistry, 58(1), 275-314. 681 doi: https://doi.org/10.2138/rmg.2005.58.11 682
Kirstein, L. A., Foeken, J. P. T., van der Beek, P., Stuart, F. M., & Phillips, R. J. (2009). 683 Cenozoic unroofing history of the Ladakh Batholith, western Himalaya, constrained by 684 thermochronology and numerical modelling. Journal of the Geological Society, 166(4), 685 667-678. https://doi.org/10.1144/0016-76492008-107 686
Kirstein, L. A., Sinclair, H., Stuart, F. M., & Dobson, K. (2006). Rapid early Miocene 687 exhumation of the Ladakh batholith, western Himalaya. Geology, 34(12). 688 https://doi.org/10.1130/g22857a.1 689
Kumar, R., Jain, A., Lal, N., & Singh, S. (2017). Early–Middle Eocene exhumation of the Trans-690 Himalayan Ladakh Batholith, and the India–Asia convergence. CURRENT SCIENCE, 691 113(6), 1090. 692
Langille, J. M., Jessup, M. J., Cottle, J., & Ahmad, T. (2014). Kinematic and thermal studies of 693 the Leo Pargil Dome: Implications for synconvergent extension in the NW Indian 694 Himalaya. Tectonics, 33(9), 1766-1786. https://doi.org/10.1002/2014tc003593 695
Laskowski, A. K., Kapp, P., & Cai, F. (2018). Gangdese culmination model: Oligocene–Miocene 696 duplexing along the India-Asia suture zone, Lazi region, southern Tibet. GSA Bulletin, 697 130(7-8), 1355-1376. https://doi.org/10.1130/b31834.1 698
Li, G., Kohn, B., Sandiford, M., & Xu, Z. (2017). India-Asia convergence: Insights from burial 699 and exhumation of the Xigaze fore-arc basin, south Tibet. Journal of Geophysical 700 Research: Solid Earth, 122(5), 3430-3449. https://doi.org/10.1002/2017jb014080 701
Li, G., Kohn, B., Sandiford, M., Xu, Z., Tian, Y., & Seiler, C. (2016). Synorogenic 702 morphotectonic evolution of the Gangdese batholith, South Tibet: Insights from low-703 temperature thermochronology. Geochemistry, Geophysics, Geosystems, 17(1), 101-112. 704 https://doi.org/10.1002/2015gc006047 705
Li, G., Tian, Y., Kohn, B. P., Sandiford, M., Xu, Z., & Cai, Z. (2015). Cenozoic low temperature 706 cooling history of the Northern Tethyan Himalaya in Zedang, SE Tibet and its 707 implications. Tectonophysics, 643, 80-93. https://doi.org/10.1016/j.tecto.2014.12.014 708
Mathur, N. (1983). The Indus Formation of the Ladakh Himalaya: its biozonation, correlation 709 and faunal provincialism. In (eds) Thakur, V.C. & Sharma, K.K. (Eds.), Geology of Indus 710 Suture Zone of Ladakh: Professor DN Wadia Birth Centenary Commemorative Volume. 711 Wadia Institute of Himalayan Geology, (pp. 127-144). 712
Najman, Y. (2006). The detrital record of orogenesis: A review of approaches and techniques 713 used in the Himalayan sedimentary basins. Earth-Science Reviews. 714 https://doi.org/10.1016/j.earscirev.2005.04.004 715
Ning, Z., Zhang, L., Huntington, K. W., Wang, C., Dai, J., Han, Z., . . . Zhang, J. (2019). The 716 burial and exhumation history of the Liuqu Conglomerate in the Yarlung Zangbo suture 717 zone, southern Tibet: Insights from clumped isotope thermometry. Journal of Asian 718 Earth Sciences, 174, 205-217. https://doi.org/10.1016/j.jseaes.2018.12.009 719
Orme, D. A. (2019). Burial and exhumation history of the Xigaze forearc basin, Yarlung suture 720 zone, Tibet. Geoscience Frontiers, 10(3), 895-908. 721 https://doi.org/10.1016/j.gsf.2017.11.011 722
Reiners, P. W. (2005). Zircon (U-Th)/He Thermochronometry. Reviews in Mineralogy and 723
Geochemistry, 58(1), 151-179. doi: https://doi.org/10.2138/rmg.2005.58.6 724
Robertson, A., & Sharp, I. (1998). Mesozoic deep-water slope/rise sedimentation and volcanism 725 along the North-Indian passive margin: evidence from the Karamba Complex, Indus 726 suture zone (Western Ladakh Himalaya). Journal of Asian Earth Sciences, 16(2-3), 195-727 215. https://doi.org/10.1016/S0743-9547(98)00010-5 728
Schlup, M., Carter, A., Cosca, M., & Steck, A. (2003). Exhumation history of eastern Ladakh 729 revealed by 40Ar/39Ar and fission-track ages: the Indus River–Tso Morari transect, NW 730 Himalaya. Journal of the Geological Society, 160(3), 385-399. 731 https://doi.org/10.1144/0016-764902-084 732
Schlup, M., Steck, A., Carter, A., Cosca, M., Epard, J.-L., & Hunziker, J. (2011). Exhumation 733 history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages. 734 Journal of Asian Earth Sciences, 40(1), 334-350. 735 https://doi.org/10.1016/j.jseaes.2010.06.008 736
Searle, M., Corfield, R. I., Stephenson, B., & McCarron, J. (1997). Structure of the North Indian 737 continental margin in the Ladakh–Zanskar Himalayas: implications for the timing of 738 obduction of the Spontang ophiolite, India–Asia collision and deformation events in the 739 Himalaya. Geological Magazine, 134(3), 297-316. 740 https://doi.org/10.1017/S0016756897006857 741
Searle, M. P. (1996). Geological evidence against large-scale pre-Holocene offsets along the 742 Karakoram Fault: Implications for the limited extrusion of the Tibetan plateau. Tectonics, 743 15(1), 171-186. https://doi.org/10.1029/95tc01693 744
Searle, M. P. (2019). Timing of subduction initiation, arc formation, ophiolite obduction and 745 India–Asia collision in the Himalaya. In P. J. Treloar & M. P. Searle (Eds.), Himalayan 746 Tectonics: A Modern Synthesis: Geological Society, London, Special Publication 483 747 (pp. 19-37). https://doi.org/10.1144/sp483.8 748
Searle, M. P., Pickering, K. T., & Cooper, D. J. W. (1990). Restoration and evolution of the 749 intermontane lndus molasse basin, Ladakh Himalaya, India. Tectonophysics, 174(3-4), 750 301-314. https://doi.org/10.1016/0040-1951(90)90327-5 751
Searle, M. P., Waters, D. J., Rex, D. C., & Wilson, R. N. (1992). Pressure, temperature and time 752 constraints on Himalayan metamorphism from eastern Kashmir and western Zanskar. 753 Journal of the Geological Society, 149(5), 753-773. 754 https://doi.org/10.1144/gsjgs.149.5.0753 755
Sharman, G. R., Sharman, J. P., & Sylvester, Z. (2018). detritalPy: A Python-based toolset for 756 visualizing and analysing detrital geo-thermochronologic data. The Depositional Record, 757 4(2), 202-215. https://doi.org/10.1002/dep2.45 758
Sinclair, H. D., & Jaffey, N. (2001). Sedimentology of the Indus Group, Ladakh, northern India: 759 implications for the timing of initiation of the palaeo-Indus River. Journal of the 760 Geological Society [London], 158(1), 151-162. https://doi.org/10.1144/jgs.158.1.151 761
Singh, I. B., Sahni, A., Jain, A. K., Upadhyay, R., Parcha, S. K., Parmar, V., . . . Ahmad, S. 762 (2015). Post-collision sedimentation in the Indus Basin (Ladakh, India): Implications for 763 the evolution of the northern margin of the Indian Plate. Journal of the Palaeontological 764 Society of India, 60(2), 97-146. 765
Steck, A. (2003). Geology of the NW Indian Himalaya. Ecologae Geologicae Helvetiae, 96, 147-766 196. https://doi.org/10.1007/s00015-003-1091-4 767
St-Onge, M. R., Rayner, N., & Searle, M. P. (2010). Zircon age determinations for the Ladakh 768 batholith at Chumathang (Northwest India): Implications for the age of the India–Asia 769 collision in the Ladakh Himalaya. Tectonophysics, 495(3-4), 171-183. doi: 770 https://doi.org/10.1016/j.tecto.2010.09.010 771
Tremblay, M. M., Fox, M., Schmidt, J. L., Tripathy-Lang, A., Wielicki, M. M., Harrison, T. M., . 772 . . Shuster, D. L. (2015). Erosion in southern Tibet shut down at approximately 10 Ma 773 due to enhanced rock uplift within the Himalaya. Proceedings of the National Academy 774 of Sciences USA, 112(39), 12030-12035. https://doi.org/10.1073/pnas.1515652112 775
Tripathy-Lang, A., Hodges, K. V., van Soest, M. C., & Ahmad, T. (2013). Evidence of pre-776 Oligocene emergence of the Indian passive margin and the timing of collision initiation 777 between India and Eurasia. Lithosphere, 5(5), 501-506. https://doi.org/10.1130/l273.1 778
Van Haver, T. (1984). Etude stratigraphique, sédimentologique et structurale d'un bassin 779 d'avant arc: exemple du bassin de l'Indus, Ladakh, Himalaya. Université Scientifique et 780 Médicale de Grenoble. https://tel.archives-ouvertes.fr/tel-00641418v2 781
Van Haver, T., Bonhomme M. G., Mascle G., and Aprahamian J. (1986), Analyses K/Ar de 782 phyllites fines des formations détritiques de l'Indus au Ladakh (Inde): Mise en evidence 783 de l'âge éocène supérieur du métamorphisme. Comptes rendus de l’Académie des 784 sciences., Serie 2, 302(6), 325-330. 785
Vermeesch, P. (2009). RadialPlotter: A Java application for fission track, luminescence and other 786 radial plots. Radiation Measurements, 44(4), 409-410. 787 https://doi.org/10.1016/j.radmeas.2009.05.003 788
Walsh, J. M. J., Buckman, S., Nutman, A. P., & Zhou, R. (2019). Age and Provenance of the 789 Nindam Formation, Ladakh, NW Himalaya: Evolution of the Intraoceanic Dras Arc 790 Before Collision With India. Tectonics, 38(8), 3070-3096. 791 https://doi.org/10.1029/2019tc005494 792
Webb, A. A. G., Guo, H., Clift, P. D., Husson, L., Müller, T., Costantino, D., . . . Wang, Q. 793 (2017). The Himalaya in 3D: Slab dynamics controlled mountain building and monsoon 794 intensification. Lithosphere. https://doi.org/10.1130/l636.1 795
Weinberg, R. F., & Dunlap, W. J. (2000). Growth and Deformation of the Ladakh Batholith, 796 Northwest Himalayas: Implications for Timing of Continental Collision and Origin of 797 Calc-Alkaline Batholiths. Journal of Geology, 108(3), 303-320. 798 https://doi.org/10.1086/314405 799
Wu, F.-Y., Clift, P. D., & Yang, J.-H. (2007). Zircon Hf isotopic constraints on the sources of the 800 Indus Molasse, Ladakh Himalaya, India. Tectonics, 26(2), TC2014 801 https://doi.org/10.1029/2006tc002051 802
Zeitler, P. K. (1985). Cooling history of the NW Himalaya, Pakistan. Tectonics, 4(1), 127-151. 803 doi: https://doi.org/10.1029/TC004i001p00127 804
Zhang, R., Murphy, M. A., Lapen, T. J., Sanchez, V., & Heizler, M. (2011). Late Eocene crustal 805 thickening followed by Early-Late Oligocene extension along the India-Asia suture zone: 806 Evidence for cyclicity in the Himalayan orogen. Geosphere, 7(5), 1249-1268. 807 https://doi.org/10.1130/GES00643.1 808
Zhou, R., Aitchison, J. C., Lokho, K., Sobel, E. R., Feng, Y., & Zhao, J.-x. (2020). Unroofing the 809 Ladakh Batholith: constraints from autochthonous molasse of the Indus Basin, NW 810 Himalaya. Journal of the Geological Society, 177(4), 818-825. 811 https://doi.org/10.1144/jgs2019-188 812
Zhu, C., Rao, S., & Hu, S. (2016). Application of illite crystallinity for paleo‐temperature 813 reconstruction: A case study in the western Sichuan basin, SW China. Carpathian 814 Journal of Earth and Environmental Sciences, 11(2), 599-608. 815
Zhuang, G., Najman, Y., Guillot, S., Roddaz, M., Antoine, P.-O., Métais, G., . . . Solangi, S. H. 816 (2015). Constraints on the collision and the pre-collision tectonic configuration between 817 India and Asia from detrital geochronology, thermochronology, and geochemistry studies 818 in the lower Indus basin, Pakistan. Earth and Planetary Science Letters, 432, 363-373. 819 https://doi.org/10.1016/j.epsl.2015.10.026 820
Figure Captions 821
Figure 1. A. Geological map of the India-Asia collision zone in Ladakh, NW India showing major tectono-822
stratigraphic units modified after Buchs and Epard (2019). Studied cross-sections are indicated in red: 1 - 823
Temesgam section; 2 - Basgo section; 3 - Zanskar Gorge; 4 - Upshi-Lato section. B: Location of the study 824
area (red) with respect to major terranes of south Asia. Blackened zones contain ophiolites. C: Schematic 825
cross-section along AA’ through the collision zone in NW India. 826
Figure 2. Geological maps of (A) Temesgam and Basgo sections (numbered 1 and 2 in red respectively; 827
modified after Garzanti and Van Haver, 1988; Tripathy-Lang et al., 2013), (B) Zanskar Gorge (modified 828
after Henderson et al. 2010), and (C) Upshi-Lato section (modified after Henderson et al. 2011) showing 829
formations, major structures and our sample locations. Abbreviations: Fm - Formation, sh - shale, 830
Conglomerate - cgl, N lst - Nummulitic Limestone, U - upper, M - middle, L - lower, R - river. 831
Figure 3. A. Plot showing range of ZFT ages from the Zanskar Gorge samples. Vertical black lines specify 832
ZFT age ranges for each sample and contain solid black diamonds that indicate corresponding depositional 833
ages. Mean percentage of grains representing modes M1, M2, M3 and M4, determined from Abanico plots, 834
are shown in parantheses. Abbreviations: Congl. - Conglomerate; Numm. Lst. - Nummulitic Limestone; n 835
- number of grains. Solid black diamonds indicate depositional ages. B. Zircon (ZHe) and apatite (AHe) 836
(U-Th)/He ages versus stratigraphic ages of the Indus Basin sedimentary rocks (IBSR). The ZHe ages (2-3 837
grains per sample) of individual grains are indicated by the horizontal bars on the dark grey rectangles. The 838
AHe ages (5 grains per sample) are represented by light grey box and whisker plots, where the whiskers 839
represent maximum and minimum individual apatite ages. Solid black squares indicate depositional ages. 840
* - The depositonal age of the Lato Formation is Late Cretaceous, which is not shown on the vertical scale. 841
The depositional ages are compiled from Bajpai et al. (2004), Wu et al., (2007), Henderson et al. (2010, 842
2011) and Bhattacharya et al. (2020). 843
Figure 4. Time-temperature (t-T) models of the Indus Basin extracted using the HeFTy program (Ketcham, 844
2005). (A) Lato Formation (Model DZA12UL), (B) Sumdo Formation (Model DZA20ZV), (C) Lower 845
Upshi Formation (Model DZA09UL), (D) Basgo Formation (Model DZA07SA), and (E) Temesgam 846
Formation (Model DZA23TM). Abbreviations: PT - paths tried, AP (green) -acceptable paths, GP (pink) -847
good paths. Solid black line indicates best fit model path. Hollow square boxes demarcate t-T constraints. 848
Table Captions 849
Table 1. Published stratigraphic schemes compared across the IBSR sections in NW India. 850
Table 2. Summary of ZFT, ZHe and AHe ages from Zanskar Gorge, Upshi-Lato, Basgo and Temesgam 851
sections. 852
Kargil
Leh
Chumathang
Upshi
Sanjak
Lamayuru
Padum
Tso Morari
Complex
Ladakh Batholith
South Tibetan D
etachment
Greater H
imalaya
Indus
River
Zanskar
River
Choksti Thrust
Nimu Thrust
Spongtang
Klippe
50 km
Great Counter
Thrust
Ribil-Zildat Fault
35°
34°
33°
76°
77°
78°77°
Tethyan Him
alaya
N
Lada
kh B
atho
lith
Indu
s Bas
in
Sed
imen
tary
Roc
ks
Dra
s C
ompl
ex
Mel
ange
ass
ocia
ted
with
Dra
s C
ompl
ex
Lam
ayur
u C
ompl
ex
Tso
Mor
ari C
ompl
ex
Thru
st fa
ult
Nor
mal
faul
t
Nid
ar O
phio
lite
Com
plex
Spo
ngta
ng
Oph
iolit
e
Indus Suture
100°80°
24°
34°
Kunlun Suture
Xigaze
Lhasa Terrane
Qiangtang Terrane
Songpan-Ganzi Terrane
Tarim Basin Qaidam BasinLeh
New Delhi
Himalaya
India
Asia
ArabianSea
Bayof Bengal
Karakoram-Baltoro
Indu
sSut
ure
YarlungSuture
BangongSuture
JinsaSuture
Temesgam
Basgo
Lato
1
2
3
4
Kailas
LadakhBatholithTethyan
Himalaya Dra
s
Lam
ayur
u
Indus BasinSedimentary Rocks
Nimu
Thrust
Cho
ksti
Thru
stGCT
AsiaIndiaNorthSouth
10 km
SpongtangKlippe
AA’
A
A’
Stu
died
sec
tion
(A)
(B)
(C)
Figure 1
(A)
Nimu Thrust (?)
10 km
Ladakh Batholith
Basgo Fm
Temesgam Fm
Lower Nimu Fm
Nurla Fm
Dras Complex
Stratigraphy
Temesgam
Basgo
LikirI
n du s
R.N 34° 15’
E 77° 10’E 77° 00’
62°
58°80°
Indu
sR
.
Lato
Upshi
(C)
Nimu Thrust
Choksti Thrust 2 km
LamayuruComplex
Lato Fm
Miru Fm
Gonmaru La Fm
Artsa Fm
Umlung Fm
Upshi Fm
Ladakh Batholith
Alluvium
N
UL
UL
Stratigraphy
N 33° 50’
N 33° 40’
E 77° 45’
55°
50°
87°
80°
60°
87°62°
Z
a
an
r R.
Jurutze Fm
Sumda Fm
Chogdo Fm
N. lst Fm
Nurla Fm
Choksti Fm
UL
U M
Cgl
Nimu Fm
Stratigraphy
1 km
N
(B)
Red sh
Choksti Thrust
Nimu Thrust
N 34° 10’
E 77° 20’
E 77° 15’
85°
85°
87°
80°
75°
60°
62°
ZHe Sample
CombinedAHe-ZHe Sample
?Contactunverified
Thrust
Town
ZFT Sample
Syncline
Anticline
Key
DZA12UL
1
2
Temesgam and Basgosections
Zanskar Gorge
Upshi-Latosection
AHe Sample
DZA09UL
DZA08UL
N 34° 10’
N
ks
DZA20ZV
DZA17ZV
ZG16
ZG21
ZG30
ZG38
ZG42
ZG55
ZG45
ZG62
DZA07SA
DZA23TM
Indus R.
Rong Fm
Figure 2
Tem
esga
m F
m
Basgo
Fm
Sumdo
Fm
Lato
Fm
5
10
15
20
25
30
35
40
45
50
55
Age (
Ma)
DZ
A23
TM
DZ
A23
TM
DZ
A07
SA
DZ
A20
ZV
DZ
A12
UL
Upp
er
Nim
u Fm
Lower
Ups
hi F
m
DZ
A17
ZV
DZ
A09
UL
DZ
A08
UL
Late Cretaceous*(not shown)Tar Group
deposition ends
Lower Indus Groupdeposition ends
(localdeposition)
Temesgam and Basgosections
Zanskar Gorge Upshi-Latosection
Tar GroupLower Indus Group
Upper Indus Group
M4 Age
(6%
)
Age (
Ma)
100
150
50
0
(37%
)(6
3%
)
Upp
er
Nim
u
(n =
45)
ZFT Results
Depositional Age
M1 Age M2 Age
M3 Age(1
2%
)(6
5%
)(2
3%
)
Lower
Nim
u
(n =
45)
(23%
)(3
7%
)(3
3%
)
Upp
er
Cho
ksti
(n =
60)
(65%
)(3
5%
)
Cho
ksti
Con
gl.
(n =
65)
(100%
)
Nur
la
(n =
17)
(6.4
%)
(38%
)(5
6%
)
Num
m.
Lst
(n =
41)
(31%
)(5
3%
)(1
6%
)
Cho
gdo
(n =
34)
(100%
)
Juru
tze
(n =
43)
Zircon (U-Th)/He age
Apatite (U-Th)/He age
Depositional age (compiled from previous studies)
(U-Th)/He Results
(B)(A)
Figure 3
35 30 25 20 15 10 5 0
Tem
pera
ture
(°C
)
240
220
200
180
160
140
120
100
80
60
40
20
0
Time (Ma)
25 20 15 10 5 0
Tem
pera
ture
(°C
)
240
220
200
180
160
140
120
100
80
60
40
20
0
Time (Ma)
(E)
Time (Ma)
25 20 15 10 5 0
Tem
pera
ture
(°C
)
240
220
200
180
160
140
120
100
80
60
40
20
0
PT: 40343AP: 398GP: 200
(A)
Time (Ma)
(C)
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Tem
pe
ratu
re (
°C)
240
220
200
180
160
140
120
100
80
60
40
20
0
PT: 456477AP: 418GP: 100
PT: 269973AP: 383GP: 100
PT: 10454AP: 217GP: 100
(B)
Time (Ma)
(D)
55 50 45 40 35 30 25 20 15 10 5 0
Tem
pe
ratu
re (
°C)
280
260
240
220
200
180
160
140
120
100
80
60
40
20
0
PT: 23201AP: 188GP: 100
Lato Formation: Model DZA12UL
Sumdo Formation: Model DZA20ZV
Lower Upshi Formation: Model DZA09UL
Basgo Formation: Model DZA07SA
Temesgam Formation: Model DZA23TM
Figure 4
Temesgam and Composite Indus Basgo sections [1] (north) (south) Group [4]
Upper Indus Local PlioceneGroup strata (f)
(Pliocene)
Temesgam Formation Upper Upshi Formation Temesgam Formation[24.6 ± 0.1 Ma]** [26.8 ± 0.1 Ma]*
Basgo Formation Basgo Formation (f)Indus Group Lower Indus [~28 Ma]#
(Early Eocene Group Rong Formation Nimu Formation– (Early Eocene Lower Upshi Formation (f) [50.0 ± 0.2 Ma]** Hemis Formation (f)
Pliocene) – [38.3 ± 0.2 Ma]** [37.8 ± 0.2 Ma]*Early Miocene) Upper Choksti Member Upper Umlung Formation
(inaccessible) Middle Choksti Member Lower Umlung Formation (f)Choksti Formation Red Shale Member (f) Artsa Formation Choksti Formation (f)
Choksti Conglomerate (f) Gonmaru La Formation (f)[41.5 ± 0.2 Ma]*
Nurla Formation Nurla Formation (f)(inaccessible) [50.7 ± 0.3 Ma]*
Marine strata
uncertainty level.
Table 1: Published stratigraphic schemes compared across the IBSR sections in NW IndiaUpshi-Lato section [3]
[52.1 ± 0.1 Ma]*
Group(Age)
Zanskar Gorge [2]
Upper Nimu Formation (f)[9.5 ± 0.5 Ma]**
Lower Nimu Formation (f)[32.3 ± 0.2 Ma]**
Nurla Formation Stratigraphy absent
Tar Group(Late Cretaceous
–
[51.8 ± 0.2 Ma]*
Maximum depositional ages are YC2σ(3+) ages, which is the weighted average of youngest 3 or more grain ages with overlapping 2σ uncertainties. All uncertainties are reported at 1σ
Jurutze Formation (f)[54.7 ± 0.3 Ma]*
Chilling Formation Lato FormationIndian margin(Cretaceous)
Note: [1] Garzanti and Van Haver (1988), Bajpai et al. (2004), Tripathy-Lang et al. (2013), [2] Wu et al. (2007), Henderson et al. (2010), [3] Henderson et al. (2011), [4] Bhattacharya et al. Symbols: * - U-Pb detrital zircon maximum depositional age, ** - 40Ar/39Ar detrital muscovite maximum depostional age, # - biostratigraphic age, f - the formation or member has a faultwith the unit immediately below or older; bold dashed line indicates unconformity.
Early Eocene)
Nummulitic Limestone
Chogdo Formation
[~50 Ma]#
Sumdo Formation[~55-51 Ma]#
Miru Formation (f)[54.9 ± 0.2 Ma]*
Section Group Formation Member Fossil Age (Ma) InterpretationYSG YC1𝜎(2+) YC2𝜎(3+) ZFT Modes
Zanskar upper Indus Upper Nimu DM: 6.1 ± 2.3 6.7 ± 1.5 9.5 ± 0.5 M1: 40.2 ± 3.1 Unreset w.r.t ZFT and ZHeGorge M2: 62.7 ± 3.5
(ZG16)
lower Indus Lower Nimu DM: 23.7 ± 0.2 32.3 ± 0.2 32.3 ± 0.2 M1: 25.5 ± 3.1 Partially reset w.r.t ZFTM2: 38.5 ± 2.7M3: 53.8 ± 5.8
(ZG21)
Upper Choksti* DZ: 36.8 ± 0.6 (H) 37.3 ± 0.3 (H) 37.8 ± 0.2 (H) M1: 26.6 ± 2.2 Partially resetDM: 37.6 ± 0.4 (LU) 38.9 ± 0.2 (LU) 38.3 ± 0.2 (LU) M2: 37.8 ± 3.8 w.r.t ZFT
M3: 49.5 ± 4.4M4: 83 ± 8.4
Choksti (ZG30)
Choksti DZ: 41.1 ± 0.3 42.0 ± 0.4 41.5 ± 0.2 M1: 40.3 ± 2.1 Partially reset w.r.t ZFTConglomerate M2: 68.6 ± 4.1 (see text for explanation)
(ZG38)
Nurla DZ: 51.0 ± 0.5 51.5 ± 0.2 51.8 ± 0.2 M1: 50.3 ± 3.3 Partially reset w.r.t ZFTDZ: 49.5 ± 0.7 (EL)** 50.2 ± 0.4 (EL) 50.7 ± 0.3 (EL) (ZG42)
Tar Nummulitic DZ: 52.5 ± 0.4 89.6 ± 0.3 90.2 ± 0.2 ~50 M1: 27.8 ± 3.5 Partially reset w.r.t ZFTLimestone M2: 62.6 ± 4.4 (see text for explanation)
M3: 91.1 ± 5.5(ZG62)
Chogdo DZ: 50.8 ± 0.5 51.3 ± 0.3 52.1 ± 0.1 M1: 22.5 ± 2 Partially reset w.r.t ZFTM2: 40.3 ± 3
M3: 63.8 ± 6.4(ZG55)
Sumdo ~55–51 Reset w.r.t ZHe
Jurutze DZ: 53.4 ± 0.7 53.7 ± 0.5 54.7 ± 0.3 M1: 65.2 ± 3.1 Partially reset w.r.t ZFT(ZG45) (see text for explanation)
Upshi-Lato lower Indus Upper Upshi DM: 24.4 ± 0.2 24.4 ± 0.2 24.6 ± 0.1
Lower Upshi DM: 37.6 ± 0.4 38.9 ± 0.2 38.3 ± 0.2 Reset w.r.t ZHe and AHe
Umlung/Artsa/Gonmaru La
Tar Miru DZ: 54.3 ± 0.4 54.7 ± 0.2 54.9 ± 0.2
Indian margin Lato DZ: 51.1 ± 0.4 481.0 ± 2.5 484.3 ± 1.7 Reset w.r.t ZHeunit DM: 67.2 ± 1.13 375.8 ± 1.0 376.3 ± 1.0
DZ***: 159.2 ± 2.0 (CF) 451.6 ± 3.9 (CF) 456.4 ± 3.4 (CF)
Basgo lower Indus Temesgam DZ: 26.2 ± 0.5 26.7 ± 0.1 26.8 ± 0.1 Reset w.r.t ZHe and AHeand
TemesgamBasgo DZ: 27.2 ± 0.5 52.6 ± 0.2 53.4 ± 0.1 ~28-26 Reset w.r.t ZHe
(DZA09UL)6.56 ± 0.10 - 5.22 ± 0.30
(DZA08UL)
12.62 ± 0.26 - 10.05 ± 0.20(DZA12UL)
18.91 ± 0.52 - 12.81 ± 0.18(DZA23TM)
6.77 ± 0.40 - 3.94 ± 0.17(DZA23TM)
Henderson et al. (2010, 2011), Bhattacharya et al. (2020).
if unreported in previous studies, were recalculated using detritalPy (Sharman et al., 2018). Details about the methods of maximum depositional age recalculation can be found in Dickinson and Gehrels, (2009).
19.04 ± 0.54 - 9.90 ± 0.27(DZA07SA)
Note: Maximum depositional ages and fossil ages are compiled from Bajpai (2004), Green et al. (2008), Wu et al. (2007), Henderson et al. (2010, 2011), and Bhattacharya et al. (2020). Youngest cluster ages, i.e., YC1𝜎(2+) and YC2𝜎(3+),
Abbreviations: YSG - youngest single grain, YC1𝜎(2+) - weighted mean age of youngest cluster of at least 2 ages with overlapping 1σ uncertainties, YC2𝜎(3+) - weighted mean age of youngest cluster of at least 3 ages with For thermochronometric interpretations, if fossil ages (bold) are unavailable for a formation, we consider the corresponding YC2𝜎(3+) age (bold), which is the most conservative estimate of maximum depositional age (Coutts et al., 2019).
*Upper Choksti Member is correlatable with the Hemis (H) and Lower Upshi (LU) Formations, and the corresponding maximum depositional ages are provided (Henderson et al., 2011; Bhattacharya et al., 2020).
**Lato Formation is also correlatable to the Indian plate Chilling Formation (CF) whose maximum depositional ages are provided (Henderson et al., 2011).
with overlapping 2σ uncertainties, DZ - detrital zircon maximum depositional age, DM - detrital muscovite maximum depositional age, w.r.t - with respect to.
Table 2. Summary of ZFT, ZHe and AHe ages from Zanskar Gorge, Upshi-Lato, Basgo and Temesgam sections. Depositional ages were compiled from stratigraphic works of Bajpai et al. (2004), Green et al. (2008), Wu et al. (2007),
**(EL) indicates the maximum depositional ages of the Nurla Formation from eastern Ladakh.
17.39 ± 0.35 - 13.70 ± 0.27
15.42 ± 0.20 - 8.57 ± 0.11(DZA20ZV)
17.79 ± 0.26 - 13.63 ± 0.21
(DZA17ZV)
Maximum Depositional Ages (Ma)Table 2: Summary of ZFT, ZHe and AHe ages from Zanskar Gorge, Upshi-Lato, Basgo and Temesgam sections
Thermochronometric Ages (Ma)ZHe Range AHe Range